UC scientists continue the quest for alternatives to farm field chemical fumigation

The first time crops and orchards are planted in virgin soil, they grow beautifully. But the plants themselves change the biology of the soil, setting up subsequent plantings for severe stress.

It is a scenario that has plagued farmers since the beginning of modern agriculture. Soil problems have been addressed over the millennia by a variety of strategies - moving to new farmland, rotating crops, leaving land fallow, and heating soil under a plastic tarp in the sun. The most effective treatment by far has been soil fumigation.

However, for the past 20 years, with increasing regulations on soil fumigants and decreasing availability of those that are most effective, UC agricultural scientists have been working on hundreds of research projects to find new alternatives. Below are some promising alternatives now being tested.

(See the sidebar for background on agriculture’s use of chemical fumigants.)

Superheated steam

Injecting superheated steam into the soil may be one way to replace chemical fumigants in agriculture. UC Cooperative Extension specialist Steve Fennimore said steam technology was already under study in California more than a century ago, but it was almost forgotten when highly effective chemical fumigation became the standard practice.

Working with a company in Kingsburg, Calif., Fennimore imported a machine that is used in Italy to prepare soil for greenhouse basil plantings. The machine, manufactured by Ferrari Constructione, has a 100-by-74-inch platform fitted with 99 ten-inch spikes that inject steam into the ground.

“It’s surprising how fast it heats the soil,” Fennimore said. “Within two minutes, it will take 60-degree soil in the surface 8 inches and heat it to 200 degrees. It’s like a microwave.”

However, there are drawbacks associated with the steam machine. Treating the field is expensive. The machine must crawl eight feet at a time up and down the field, each time pushing the spikes into the ground and leaving them positioned there for six minutes while the steam does its work. With one steam machine, it takes about 30 hours per acre.

“We tested a small machine that was easy to ship and most accessible,” Fennimore said. “It is not the final design by any means.”

Fennimore calculated that operating the steam machine, labor and fuel cost $3,848 per acre. Currently, chemical fumigation in California costs $2,700 to $3,000 per acre. Applying steam to raised beds rather than the entire field could cut steam cleaning expenses to about $3,000 per acre.

Another concern is air quality. The Italian machine runs on diesel fuel. In some parts of California, the operation of diesel engines is regulated to reduce air pollution.

“We’re concerned about the air quality issue and people point out all the time that we’re burning lots of carbon,” Fennimore said. “That’s true, but you can only solve so many problems at once. We’re trying to develop a practical system and we’re interested in switching to propane.”

Fennimore is encouraged by the success Italians have had with the steam system.

“The Italians have not been able to use (the fumigant) methyl bromide for a while now,” Fennimore said. “Yet, they have a number of the same industries that we have and they’re able to keep going.”

Using steam to treat land before replanting orchards and vineyards is another issue. The 10-inch depth of the steam injectors wouldn’t be sufficient for trees whose root zones reach four to six feet into the ground. As a gas, methyl bromide and other fumigants readily move through tiny gaps in soil and with proper application can treat deep down under the soil surface.

For orchard and vineyard replant purposes, UC Riverside nematologist Mike McKenry is studying steam application, not on the whole field, but in holes that give steam access to the full root zones of future trees. In his research, McKenry is using a steam generator powered by propane with an injector on a long hose.

“We will pull the steam generator on a tractor, there’s an auger on the front of it, we will dig down four feet and we will treat with steam prior to the planting of the tree,” McKenry said. “We believe it will provide a brief period of relief from soil pests and diseases.”

Starve and switch

McKenry is also studying a fumigation alternative he named “starve and switch.”

While orchards grow, sometimes for decades, the chemicals excreted by the tree roots alter the flora and fauna in the soil. The resulting soil environment will severely hinder the growth of new trees when the orchard is replaced, a process McKenry calls “the rejection component.” The rejection component, McKenry said, inhibits the tree growth for at least a year after replanting the tree, after which, if no other problems are present, the trees may grow as if they had been planted in fumigated soil. But that means the farmer must wait longer to get a return from the investment.

Sanitizing the soil with a fumigant works remarkably well in correcting this problem. It takes care of nematodes, plant diseases, weed seeds, the rejection component and it even causes nitrogen in the soil to be made available, giving the tree a good start. But with fumigants on the way out, McKenry developed starve and switch.

“We kill the old roots with an appropriate systemic herbicide such as Roundup and fallow the field for a year so the microbes in the soil starve,” McKenry said. “Then we switch to a rootstock with tolerance to the rejection component.”

In a replicated trial of almond trees planted in spring 2009 -- where some sections were treated before planting with the fumigant Telone, other sections subjected to starve and switch, and other sections planted with no special treatment -- the trees’ canopies tell the story. One year after they were planted, the Telone trees are dense and tall, the starve and switch trees nearly as tall but with much less foliage, and the untreated tree decidedly smaller and less dense.

New rootstocks

Replanting walnut orchards without fumigation has been notoriously difficult for farmers since there are no nematode-resistant rootstocks. But there is hope on the horizon. One tree from China – Juglans cathayensis No. 21 – has walnut breeders excited.  It has natural resistance to root knot and root lesion nematodes.  For the first time, a rootstock may be available in walnuts to try starve and switch, the process in which trees are killed with Garlon herbicide, the field left fallow for a year and then a tree on resistant rootstock is replanted.

McKenry has already tried grafting an English walnut to the rootstock and it takes the graft, however, at the graft union, there is a lack of compatibility, resulting in what is likely a weak connection that could break when trees are shaken for harvest.

“This rootstock will have to go back to Davis and be crossed with English walnut to bring improved graft affinity to the graft union,” McKenry said.

Sidebar: Background on soil fumigants

Since the mid 20^th century, farmers have routinely used fumigants to prepare soil for sensitive annual crops, like strawberries and vegetables, and for planting new orchards on land where previous orchards or vineyards of the same kind were grown.

Several of the most effective fumigants – such as methyl bromide, chloropicrin and Telone – work so well, scientists can’t even fully explain why the crops are so much more productive after treatment.  But now, the use of soil fumigants is falling out of favor.

Soil fumigation was first used in the late 1880s when the grape root aphid Phylloxera was exported from the Eastern United States to Europe, with devastating effect. In desperation, farmers tried to control the pest with carbon disulfide, a readily ignitable chemical that presented explosive danger to workers applying the pesticide.

By the 1930s, 17 chemicals were available to farmers to control nematodes, tiny soil-borne roundworms that harm crops. In 1941, USDA scientists Al Taylor and C.W. McBeth were the first to report a practical application for the chemical compound methyl bromide in agricultural fields. Methyl bromide is a naturally occurring, but toxic compound, produced by the earth’s oceans, volcanoes and wildfires.  It can also be manufactured.

It would be almost two decades before UC Berkeley plant pathologist Stephen Wilhelm and businessman William Storkan developed a method for applying the gas to an agricultural field while at the same time spreading a polyethylene tarp onto the field surface to keep the fumigant in place.

Methyl bromide treatment solves many agricultural problems. It kills nematodes, fungi, plant pathogens and weed seeds. When properly applied, methyl bromide moves freely through tiny gaps in the soil, creating an optimum environment for plant root growth and development.  Treated soil permitted plants to reach their full potential, increasing yields by dramatic proportions. In strawberries, for example, yields more than quadrupled from the early 1960s to the 1990s.

However, as the turn of the millennium approached, climate scientists found that the productivity came with a price. Methyl bromide is an ozone-depleting chemical. Its production and use worldwide were severely curtailed by the Montreal Protocol and its use as an agricultural pesticide is being phased out by most countries, including the United States. The agricultural industry has petitioned for extensions of the methyl bromide ban because it lacks an equally effective alternative. Unfortunately, many chemical alternatives have been found to be highly toxic, which has resulted in strict regulations for their use – requiring wide buffer zones, careful timing, worker safety measures, etc. In many cases, the use of chemicals is impossible.

The reduced availability of soil fumigants in agriculture is prompting University of California scientists to look for alternatives.